This application is 371 of PCT/JP98/03569 filed on Aug. 10, 1998.
This invention relates to a cycloalliin-enriched onion extract and a process for producing the same.
It is known that cycloalliin is a cyclic sulfur-containing amino acid having bioactivity such as fibrinolytic, hypoglycemic and hypolipidemic actions [Atherosclerosis, 21, 409-416 (1975), Japanese Laid-open H5-194237]. The onion intracellularly and abundantly contains the cycloalliin precursors S-(1-propenyl)-L-cysteine sulfoxide (hereinafter referred to as xe2x80x9cprecursor Axe2x80x9d) and xcex3-glutamyl-S-(1-propenyl)-L-cysteine sulfoxide (hereinafter referred to xe2x80x9cpeptide precursorxe2x80x9d). It is known that said peptide precursor is transformed to said precursor A by the enzyme xcex3-glutamyl peptidase, xcex3-glutamyl transpeptidase or the like (Advances in Food Research, 22, 73-133 (1976)) and that precursor A is converted to cycloalliin on heating or alkali treatment (Bioscience, Biotechnology and Biochemistry, 58, 108-110 (1994)). 
The onion extract heretofore available is intended for use as a condiment; because of a unique flavor originating from its volatile sulfur-containing fraction (disulfide, etc.), the onion extract has been used as a very important food component for seasoning. Production of such an onion extract for use as a condiment involves a sequence of peeling and washing onions, crushing and squeezing them, concentrating the resulting juice under reduced pressure, and pasteurizing the product. The onion flavor is expressed as said precursor A is converted to sulfenic acid by the CS-lyase endogenously present in onions in the crush-squeezing stage and the sulfenic acid so produced is then transformed through the thiosulfinate to the disulfide which is a flavor source.
Thus, in the process for producing an onion extract for use as a condiment, said precursor A is converted to the disulfide so that the conventional onion extract is lean in cycloalliin.
It is an object of this invention to provide an onion extract having an increased cycloalliin content (containing not less than 0.1%, preferably not less than 0.3% of cycloalliin as Brix 70 equivalent) which is of value as a health food or a starting material for the production of health foods and the other object is to provide an efficient process for producing a cycloalliin-containing onion extract which is easy to handle.
To accomplish the above objects, the inventors of this invention did many investigations. As a result, they discovered that when the CS-lyase is inactivated by heating onions prior to the crush-squeezing step, the loss of said precursor A is reduced and that when the onion juice available on crushing and squeezing of heated onions is further heated or subjected to alkali treatment, the precursor A is successfully converted to cycloalliin. This invention has been accordingly completed.
This invention, in one aspect, is directed to an onion extract rich in cycloalliin. In another aspect, this invention is concerned with a process for producing a cycloalliin-containing onion extract characterized by its comprising heating onions in the first place, crushing and squeezing them, and subjecting the resulting onion juice to reheating or alkali treatment.
The variety, growing district and harvest season of the onions to be used in this invention are not particularly restricted. The method of storage of the onions is not particularly restricted, either.
The onion juice in the context of this invention means a juice obtainable by the process which comprises crush-compressing onions by a suitable method and subsequently centrifuging them or extracting the same with, for example, water or warm water. The onion extract means a preparation obtainable by reducing the water content of the onion juice by a suitable method to thereby increase the percentage of solid matter.
The onion extract enriched in cycloallliin according to this invention can be produced by carrying out the following sequence of steps.
(1) Pretreating step (washing, peeling, sterilization, etc.)
(2) First heating step
(3) Crush-squeezing step
(4) Second heating or alkali treatment step
(5) Concentration step
(6) Pasteurization step
The cycloalliin content of the extract can be further increased by interposing a step of treating the onion juice with an enzyme having peptide precursor (xcex3-glutamyl peptide)-cleaving activity [xcex3-glutamylpeptide cleavage step] prior to said (4) second heating or alkali treatment step.
This invention is now described in detail.
(1) Pretreating Step (Washing, Peeling, Sterilization, etc.)
Onions are washed with water using a water washing machine or the like, followed, if necessary, by peeling, sterilization, etc. The machinery or equipment for water washing is not particularly restricted.
(2) First Heating Step
The onions are then heated to inactivate the CS-lyase (first heating step). The heating temperature is 60-120xc2x0 C., preferably 70-100xc2x0 C., and the heating time is one minute to one hour, preferably 5-30 minutes.
(3) Crush-squeezing Step
The onions are then crushed to suitable size. The machinery or equipment for crushing is not particularly restricted. For example, a pulverizing machine such as a feather mill, a homogenizer such as Mascolloider (Masuko Sangyo), a dicer, a chopper, or a home food cutter, for instance, can be mentioned. With an instant heater-disintegrator or the like machine which achieves both steam inactivation of the enzyme and crushing, the first heating and crushing operations can be more efficiently carried through.
To prepare an onion juice, the crushed onions are squeezed with a suitable press machine or centrifuged with a centrifugal machine. As an alternative, the crushed onions are extracted with hot water. For improved squeezing efficiency in such a hot-water extraction process, an enzyme treatment with cellulase, pectinase or protease etc. can be performed. The enzyme to be used is not particularly restricted insofar as it contributes to an improved squeezing efficiency. Usually, such an enzyme is added to water in a proportion of 0.005-0.1 weight % relative to the onions and the onions are treated under warming at pH 2-9 and 40-70xc2x0 C. for 30 minutes to 40 hours.
(4) Second Heating or Alkali Treatment Step
Then, for the purpose of converting precursor A to cycloalliin, the onion juice is reheated or treated with an alkali. The temperature for reheating is 90-120xc2x0 C., preferably 90-100xc2x0 C., and the heating time is 10 minutes to 4 hours, preferably 30 minutes to 2 hours. The alkali treatment is carried out at pH 7-12, preferably pH 9-10, and the treating time is 5 minutes to 2 hours, preferably 10 minutes to 1 hour. The alkali for use is not particularly restricted but includes sodium hydroxide, potassium hydroxide, sodium carbonate, potassium carbonate, sodium hydrogencarbonate and potassium hydrogencarbonate, among others. When the product after the alkali treatment is alkaline, it is neutralized with an acid such as hydrochloric acid. The alkali treatment and heat treatment can be carried out in combination.
The heating operation for inactivating said CS-lyase (first heating step) and the heating operation for converting precursor A to cycloalliin (second heating step) may be performed in a single heating cycle. In this case, however, the onions will be softened too much to retain their shape so that considerable difficulties will be encountered in their transportation to the crush-squeezing equipment and the crushing and squeezing operation there. The result is that an onion extract can hardly be produced on a mass scale with good efficiency. The above difficulties can be obviated by carrying out heating in two steps in accordance with this invention.
xcex3-Glutamyl Peptide-cleaving Step
Prior to said (4) second heating or alkali treatment step for converting precursor A to cycloalliin, the onion juice is treated with an enzyme having xcex3-glutamyl peptide-cleaving activity, whereby peptide precursor can be converted to precursor A. As a result, the cycloalliin content of the onion extract can be further increased.
Thus, to the juice obtained by said first heating and crush-squeezing operations, an enzyme having xcex3-glutamyl peptide-cleaving activity is added and the juice is treated around the optimal pH of the enzyme used, for example pH 2-9, at a temperature of 40-50xc2x0 C. for 2-4 hours. The level of use of the enzyme is 0.001-1.0 weight % relative to the onion juice. After this treatment, the enzyme is inactivated by heating at its inactivation temperature, for example heating up to 80-90xc2x0 C.
The enzyme for use is not particularly restricted insofar as it has xcex3-glutamyl peptide-cleaving activity, thus including xcex3-glutamyl peptidase, xcex3-glutamyl transpeptitase and glutaminase, among others. As the enzyme, the fermentation product obtained or derived by growing a producer strain of microorganism in the routine manner or the enzyme which can be derived from an animal tissue (kidney, small intestinal mucosa, etc.) or isolated from the vegetable kingdom (e.g. mushrooms such as Cortinellus shiitake, meadow mushroom, etc., asparagus, kidney bean, etc.). As an alternative, a commercial enzyme or an enzyme purified from a commercial enzyme or enzyme preparation can also. be utilized.
Furthermore, at the stage of converting peptide precursor to precursor A using said enzyme having xcex3-glutamyl peptide-cleaving activity, the onion juice may be optionally treated with such an enzyme as pectinase, cellulase or the like for the purpose of reducing the viscosity of the onion extract in the subsequent concentration step. The enzyme which can be used for this purpose is not particularly restricted insofar as it is capable of reducing the extract viscosity. The level of addition of this enzyme is generally 0.005-0.1 weight % relative to the onion juice. The conditions of this enzymatic treatment vary with different kinds of enzymes but may for example be 40-50xc2x0 C., pH 2-9, and reaction time 2-4 hours. After this enzyme treatment, the enzyme is generally inactivated by heating at the inactivation temperature of the enzyme used, for example heating up to 80-90xc2x0 C.
(5) Concentration Step
The onion juice is concentrated to provide an onion extract. The method for concentration includes the vacuum concentration method using an evaporator, the method using a concentration kettle, the method using a reverse osmosis membrane, the freeze-concentration method and any other method. The concentration equipment suited to each method can be employed. Thus, provided that the water content can be reduced, any equipment can be utilized. The degree of concentration is not particularly restricted but is preferably within the range of 30-75 weight % in terms of the concentration of water-soluble solids in the concentrate.
(6) Pasteurization Step
The onion extract thus obtained is usually heated for sterilization. The pasteurizing conditions are preferably as mild as possible in order to prevent deterioration in color and flavor. Specifically, the conditions of 90-95xc2x0 C. for 5-60 minutes or 100-130xc2x0 C. for 3-50 seconds may be mentioned. The onion extract thus obtained contains cycloalliin in a proportion of not less than 0.1% in terms of Brix 70 equivalent (Brix is a unit representing the concentration of water-soluble solids and Brix 70, for instance, means that the concentration of soluble solids is 70%; the same applies hereinafter).
The cycloalliin-rich onion extract of this invention is of value as a health food or a raw material for health foods and can be put to use either as it is as a food or by adding it to other foodstuffs.